Display the awaiter call stack in Observatory
Display the awaiter call stack in Observatory

BUG=
R=rmacnak@google.com

Review-Url: https://codereview.chromium.org/2785553005 .

dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk